1. Field of the Invention
The present invention relates to position detectors and position indicators therefor for use in inputting graphics and characters to computers.
2. Description of the Related Art
One type of known position detector is disclosed in Japanese Unexamined Patent Application Publication No. Kokai Hei 1-53223 (earlier patent application 1)(U.S. Pat. No. 5,134,689).
In this position detector, a position indicator is provided with a resonant circuit containing a coil and a capacitor. Electromagnetic waves are transmitted between a tablet and the position indicator, whereby coordinate values of a position indicated by the position indicator are detected. Of a plurality of loop coils which are provided in the tablet and which are parallel to position detecting directions, the loop coils are sequentially selected, and the selected loop coil emits electromagnetic waves. The electromagnetic waves which are reemitted from the resonant circuit in the position indicator are received, and coordinate values of the position indicated are thereby detected on the basis of a distribution of reception signal strengths.
In this type of position detector, there are demands for inputting, in addition to the coordinate values of the position indicated, information which indicates the state in which the position indicated (which should be actually input) is specified. Examples of such information include information for continuously varying the line thickness, hue, and luminance.
In the earlier patent application 1, the coil which forms the resonant circuit is a coil whose inductance continuously varies in accordance with the writing force. Thus, the resonance frequency continuously varies in accordance with the writing force. A continuous variation in the resonance frequency is detected as a continuous variation in phase angle, and the writing force is detected thereby.
The range of phase angles, which corresponds to the range of writing forces to be detected, varies as a result of factors such as the distance from the tablet to the position indicator, the tilt of the position indicator with respect to the tablet, etc. Also, the range of phase angles (writing forces) varies in accordance with the aging of the inductance of the coil. It is thus impossible to accurately detect writing forces.
In Japanese Unexamined Patent Application Publication No. Kokai hei 7-225644 (earlier patent application 2), the applicant of the present invention has proposed a pen in which the writing force and the continuous quantities of the three RGB colors are set, thereby allowing information such as color hue and color strength to be input with the pen. More specifically, four variable capacitors are provided in order to set the writing force and the quantities of three colors. At four different times, the four variable capacitors are sequentially connected to the resonant circuit, whereby the four continuous quantities are independently detected.
With the foregoing pen, the phase of a signal when no variable capacitors are connected is detected beforehand, thereby suppressing effects caused by the aging of the coil inductance as in the earlier patent application 1. In contrast, as in the earlier patent application 1, the range of phase angles corresponding to the range of continuous quantities to be detected varies as a result of factors such as the distance from the pen to the tablet, the tilt of the pen with respect to the tablet, etc. It is thus impossible to obtain the correct continuous quantities.
The applicant of the present invention has proposed in Japanese Unexamined Patent Application Publication No. 5-313439 (earlier patent application 3)(U.S. Pat. No. 5,679,930) a position indicator which converts therein a continuous quantity into binary code and which transmits back the binary code to the tablet, thereby detecting the continuous quantity.
With this position indicator, an accurate continuous quantity can be detected while factors such as the height and tilt of the position indicator with respect to the tablet have no influence on the position indicator. In contrast, the structure of the position indicator becomes complex. The position indicator has another disadvantage in that the sampling rate decreases due to the necessity for repeating transmission/reception for each bit because a single continuous quantity is transmitted back in terms of binary code.